1. Field of the Invention
The invention relates to anionic, hydrotalcite-type pillared clay compositions and their heat-treated derivatives. The invention also relates to a process for reducing the sulfur oxide content of a gaseous mixture by absorbing sulfur oxides on an absorbent which can be reactivated for further absorption through contact with a hydrocarbon in the presence of a hydrocarbon cracking catalyst.
2. Description of the Prior Art
The development of efficient methods and catalysts for reducing the concentration of air pollutants, such as sulfur oxides, in gaseous mixtures which result from the processing and combustion of sulfur-containing fuels presents a major industrial problem which has interested researchers for a considerable time. For example, U.S. Pat. No. 3,835,031, issued to Bertolacini et al. and assigned to the assignee of the present application, describes a cyclic, fluidized catalytic cracking process operating with a catalyst comprising a molecular sieve in a silica-alumina matrix which is impregnated with one or more Group IIA metal oxides, such as magnesium oxide. By absorbing sulfur oxide within a regeneration zone and, subsequently, releasing the absorbed sulfur within a cracking reaction zone, emission of sulfur oxides in a regenerator stack gas stream is greatly reduced.
Other researchers have noted that absorbents containing rare earth metals are suitable for sulfur oxide removal service. U.S. Pat. No. 4,146,463, issued to Radford et al. and assigned to the assignee of the present invention, describes the absorption of sulfur oxides by modified catalyst particles containing the oxides of rare earth metals, such as cerium, lanthanum and neodymium. The modified catalyst particles reportedly form non-volatile sulfur compounds by reacting with sulfur oxides in a regeneration zone.
Researchers have attempted to identify an optimal structure for sulfur oxides separation catalysts. U.S. Pat. No. 4,626,419, issued to Lewis et al., is directed to a composition of matter for removing sulfur oxides from gases which comprises an alkali metal and a crystalline rare earth oxide, such as cerium oxide, having a crystal size of less than about 90 Angstrom units. The '419 Patent states that improved results measured as a reduction of sulfur in regenerator off-gas may be obtained using oxide crystals in the specified size range.
Sulfur oxide separation catalysts containing magnesium and aluminum crystalline structures in spinel form are reported, for example, in U.S. Pat. No. 4,790,982, issued to Yoo et al., which describes the use of a magnesium and aluminum spinel in conjunction with cerium metal and free magnesia. U.S. Pat. No. 4,728,635, issued to Bhattacharyya et al., is directed to a process for the production of a calcined alkaline earth, aluminum-containing spinel composition for use as a sulfur oxide and nitrogen removal agent.
U.S. Pat. No. 4,865,019, issued to Van Broekhoven, describes sulfur-oxide absorbents which comprise an anionic clay having a hydrotalcite structure. The '019 Patent states that the anionic clay can have a layered structure corresponding to a formula calling for divalent cations, trivalent cations, and anions in specified proportions. Preference is given to divalent cations Mg.sup.2+ and trivalent cation Al.sup.3+ alone or combined with La.sup.3+ and/or Ce.sup.3+. Anions NO.sub.3-, OH-, CI-, Br-, I-, CO.sub.3.sup.2-, SO.sub.4.sup.2-, SiO.sub.3.sup.2-, SiO.sub.3.sup.2-, CrO.sub.4.sup.2-, HPO.sub.4.sup.2-, MnO.sup.4-, HGaO.sub.3.sup.2-, HVO.sub.4.sup.2-, CIO.sub.4.sup.2-, BO.sub.3.sup.2-, monocarboxylates, dicarboxylates, alkyl sulfonates, and combinations thereof are listed as suitable. The '019 Patent states that the absorbents are useful after a heat treatment to a temperature in the range of about 300.degree. to about 900.degree. C. which reportedly can involve some decomposition of the hydrotalcite structure.
U.S. Pat. No. 4,774,212, issued to Drezdon and assigned to the assignee of the present invention, describes magnesium and aluminum hydrotalcite-type clay compositions having polyoxometalates of vanadium, tungsten, or molybdenum as pillaring anions. The compositions are reported to have an x-ray diffraction d(003) value which is larger than that of typical hydrotalcites, indicating a greater spacing between clay layers. Reference is made to use of the compositions for catalysis at temperatures in the range of about 200.degree. to about 600.degree. C. The '212 Patent also presents a method of preparing the described compositions which involves formulating hydrotalcite-like clays pillared by relatively large organic anions and replacing the organic anions with polyoxometalates from a solution having a pH of about 3 to about 6.
A direct and relatively simplified process for making anionic clays having a hydrotalcite structure pillared by pH-dependent inorganic anions is set forth in U.S. Pat. No. 5,246,899, issued to Bhattacharyya and assigned to the assignee of the present invention. In a preferred aspect, the process comprises adding a solution containing one or more selected divalent cations and one or more selected trivalent cations to an essentially carbonate-free solution which includes an inorganic ion and has a selectively chosen pH between about 6 and 14.
U.S. Pat. No. 5,288,675, issued to Kim, contemplates a MgO/La.sub.2 O.sub.3 /Al.sub.2 O.sub.3 ternary oxide base wherein the MgO component is present as a microcrystalline phase which may be detected by x-ray diffraction. The ternary oxide base can reportedly be used in combination with ingredients such as ceria and/or vanadia to control sulfur oxide emissions. The '675 Patent states that the combination can be prepared by a multi-step process which includes reacting an aged, coprecipitated lanthanum and aluminum hydrous oxide slurry with a magnesium oxide slurry and a sodium hydroxide solution, calcining, impregnating with solutions of cerium and/or vanadium and calcining at a temperature of 450.degree. to 700.degree. C.
Sulfur oxide emissions from fluid catalytic cracking units, for example, are increasingly restricted by environmental regulations. The removal of sulfur oxide pollutants has been the subject of considerable attention for several years. One approach to reducing such emissions involves desulfurizing a hydrocarbon feed stream before it enters the cracking unit, so that a lesser amount of sulfur oxides are produced. Another approach is to scrub the emissions stream with an inexpensive alkaline material, such as lime or limestone. However, both of these approaches are relatively cumbersome and they create other waste disposal problems. Accordingly, separating the sulfur oxides by contact with a reusable absorbent presents an appealing alternative.
It is generally accepted that sulfur trioxide (SO.sub.3) absorption proceeds more rapidly than sulfur dioxide (SO.sub.2) absorption. Accordingly, efficient sulfur dioxide absorbents must perform at least three functions. First, desirable absorbents have a catalytic capability that allows them to enhance the reaction of sulfur dioxide with oxygen to form sulfur trioxide. Second, desirable absorbents are capable of binding sulfur trioxide in relatively large amounts. Third, desirable absorbents can desorb sulfur components comparatively quickly on exposure to hydrocarbons and cracking catalyst.
The sulfur oxide absorbents which have received the widest commercial acceptance to date in fluidized catalytic cracking units are based on spinel technology, most notably MgAl.sub.2 O.sub.4 spinels combined with cerium oxide. Although the spinel and cerium absorbents are adequate for many purposes, they exhibit limited absorbent capacity and are prone to deactivation. In particular, free cerium oxide crystals present in the spinel and cerium absorbents tend to increase in size during normal operation so as to inhibit overall activity. Additionally, the spinel and cerium absorbents require more time for complete desorption than is available in some cyclic processing schemes.
Accordingly, a need still exists for new absorbents which can absorb and desorb comparatively larger amounts of sulfur compounds per unit mass within relatively short cycle time periods. Catalytic materials on the absorbents must be well dispersed for maximum accessibility and resist the tendency to agglomerate under operating conditions. Additionally, the absorbents should resist physical attrition and demonstrate superior stability at processing temperatures in both oxidizing and reducing environments.